Tested with Catalyst 13.12 on a HD 6850. When using glCopyImageSubdata with both the source and destination having a compressed internal format ( NOTE: The source and destination texture were different in my test case ), a GL error is generated. If the source and destination format is not uncompressed, then there is not GL error. I retested using the glCopyImageSubDataNV ( this is implemented by the AMD driver ) and there is not GL error. I'm assuming the behavior of the 2 functions should be similar with the exception that glCopyImageSubDataNV is more restrictive as noted on the extension description page.